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158                                ALASKAN GLACIER STUDIES
The rate of advance cannot be determined exactly, but the following tentative computation can be made. If it had taken the entire period of a little less than three years, from July 10, 1906, to July 6, 1909, for the ice front to advance the two miles, the rate of forward movement of the ice front, ignoring wasting by melting, would have been nine and seven-tenths feet a day, or faster than Muir Glacier was moving in 1890 when its seven feet of daily movement were compensated for by iceberg discharge from the front. The two mile advance of Hidden Glacier, however, we know to have occurred not in three years, but probably in less than one year and, judging by the known advance of Variegated, Haenke, Atrevida, and Marvine Glaciers, during a very few months. Its rate of forward motion must have been several times the nine and seven-tenths rate, and may even have attained a rate of 30 to 50 feet a day. Although we cannot give the rate of advance exactly, it is easy to see that the advance of Hidden Glacier was far different from normal glacier movements and may be spoken of as a spasmodic rush, or flood.
In connection with the recent advance of Hidden Glacier it is of interest to note that one of its through glacier connections on the Alsek may also have commenced to advance
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in 1909. The Boundary Survey party which ascended the Alsek in 1908 reported no activity of any glacier on the west side of the river that year. About the first of August, 1909, however, as we learned from the Dry Bay natives, the Alsek river rose 16 or 18 feet on its delta, maintaining that level for at least two weeks. The waters carried icebergs, which was unusual, and big tree trunks, and forced the natives to move from their village. This rise is interpreted as meaning the crevassing of some glacier up the river, and hence greater melting. The glacier is thought to be one above the lower Alsek canyon, below which no large trees grow, and may well be the large ice tongue on the west bank, next above the lower Alack Canyon, which connects with the Yakutat, Hidden, and Nunatak Glacier systems. Since equilibrium is disturbed in this glacier system, as evidenced by the two mile advance of Hidden Glacier, it is quite likely that an advance has also occurred on the Alsek, and that other advances may affect the other glaciers of Alsek valley, the Yakutat Glacier and the smaller ice tongues within the next year or two, as it did Nunatak Glacier in 1910.                                           .              -
Conditions in 1910. The junior author found in June, 1910, no particular changes in Hidden Glacier since the year before except the continuation of retreat and thinning by ablation. The drainage had not altered significantly,
Detailed studies of Seal Bay, the submerged continuation of the Hidden Glacier valley,